If you have a level 70 alt. Great. Can always be handy both for yourself to have a change, and in case we need set classes.
Note however we can only accept one as your main. If you desire to change that, then you will need to go through the trial procedure again. This is to protect the guild from losing someone who is good at one class for an alt they are less good on.
Also can only really do this if it is ok to lose a member from Class A, and them swap to Class B.